1. Field of the Invention
The invention relates generally to the field of seismic exploration. More particularly, the invention relates to methods for acquiring marine seismic data using selected arrangements of sources and receivers.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in the ocean, one or more air guns or water guns.
Seismic energy emanates from the source and travels through the earth formations until it reaches an acoustic impedance boundary within the earth formations. Acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formations change. At an acoustic impedance boundary, some of the seismic energy is reflected back toward the earth's surface, where it may be detected by one or more of the seismic sensors deployed on the surface. Other portions of the seismic energy are refracted and continue propagating in a generally downward direction until another acoustic impedance boundary is reached. Seismic signal processing known in the art has as an objective the determination of the depths and geographic locations of bed boundaries below the earth's surface from signals related to the reflected acoustic energy. The depth and the location of the bed boundaries is inferred from the travel time of the seismic energy to the acoustic impedance boundaries and back to the sensors at the surface.
Seismic surveying is performed in the ocean (and other large navigable bodies of water) to determine the structure of earth formations below the sea bed (or water bottom). Marine seismic surveying known in the art includes having a vessel tow one or more seismic energy sources, and the same or a different vessel tow one or more “streamers”, which are arrays of seismic sensors forming part of or otherwise affixed to a cable at spaced apart locations along the cable. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected lateral distance from each other, in a pattern designed to enable relatively complete determination of subsurface geologic structures in three dimensions.
The signals detected by the seismic sensors at the earth's surface (or near the water surface) include components of seismic energy reflected at the bed boundaries, as previously explained. In addition, both coherent noise (noise which has a determinable pattern, such as may be caused by a ship propeller) and incoherent (random) noise may be present. The presence of such noise in the signals received by the seismic sensors reduces the signal-to-noise ratio (“SNR”) of the seismic signals of interest. An objective of seismic signal processing is to substantially eliminate the effects of noise on the signals detected by the sensors without appreciably reducing the reflected seismic energy component of the detected signals.
Prior art methods which have been used to reduce the effects of noise and acquire a higher quality seismic representation of subsurface structures include using multiple actuations of the seismic energy source (multiple “firings” or “shots”) to record a plurality of sensor measurements from substantially the same subsurface structure, and then summing or “stacking” such measurements to enhance signal strength while substantially reducing the effects of random or incoherent noise.
U.S. Pat. No. 5,818,795, which is assigned to the assignee of the present invention, provides a detailed summary of prior art methods and systems addressing the problem of noise elimination in seismic signals, and discloses as well a method of reducing the effect of “burst” noise in seismic signal recordings without eliminating signals related to reflected seismic energy.
In marine seismic surveying, it is known in the art to increase the effective subsurface length of coverage of a seismic streamer by using an additional seismic energy source at a spaced apart position along the survey line (direction of travel of the seismic vessel). The additional seismic energy source may be towed ahead of or behind the vessel that tows the other source and/or the seismic streamer(s). Generally speaking, methods known in the art include firing the first source and recording signals resulting therefrom, waiting a selected delay time to allow seismic energy from the first source to attenuate, and then actuating the second source. U.S. Pat. No. 5,761,152, which is assigned to the assignee of the present invention, describes a method and system for marine seismic surveying which increases the fold (number of recorded reflections from a same reflector), and hence the signal-to-noise ratio of seismic signals, without incurring the problems of drag, entanglement, complicated deck handling, and decreased signal-to-noise ratio associated with increased streamer length, increased number of streamers, and increased distance between streamers. Source and streamer “offsets”, and time of firing of lead and trailing vessel sources in a time delay sequence are optimized to increase the fold while avoiding any influence by the seismic signals resulting from the source of one vessel on the seismic signals resulting from the source of the other vessel.
A limitation to methods known in the art for using more than one seismic source, such as disclosed in the '152 patent, for example, is that it is necessary to wait a substantial amount of time, typically several seconds or more, between firing the first source and firing the second source, to enable identification of the energy in the recorded seismic signals as having been caused by the first or the second source. Such identification is necessary in order to properly interpret subsurface structures from the detected seismic signals. The waiting time between firing the first source and the second source reduces the speed at which seismic surveys can be recorded, and thus reduces the efficiency of making such surveys. Accordingly, it is desirable to be able to reduce the waiting time in multiple source seismic surveys to a minimum.